Many metamorphic mineral reactions have been investigated experimentally using hydrothermal apparatus and it became evident that the amount and the composition of the fluid phase is important for the kinetic and the mechanism of the overall reactions. The diopside-forming reaction
1 dol + 2 qtz = 1 di + 2CO2
was subject of intensive investigation by Lüttge and Metz (1991, 1993) and Neumann and Lüttge (subm.). Results from their numerous powder, single crystal and rock cylinder experiments at high temperatures and pressure (640-680°C, 500 MPa) indicate that the initial stage of a heterogeneous mineral reaction is an important key for the understanding of its mechanism and kinetics. Sequential and/or parallel processes during this initial stage can strongly influence the composition of the fluid phase and accordingly the rate controlling mechanism. Unfortunately, these processes are not well understood yet and even complicated by several experimental factors.
Different methods in heating-up a conventional hydrothermal apparatus (externally/internally heated) produce variable results due to different times in which equilibrium and run temperature are reached. A fast heating-up procedure (within minutes or less than one minute) will generally result in lower and/or different initial concentrations of the dissolved species in the fluid phase when reaching the reaction temperature. Thus, a steady-state will establish earlier or later during the experiment resulting in different conversion versus time data. In addition, metastable minerals (in view of the reaction studied) formed below the equilibrium temperature of the stable product(s) should be suppressed in light of the short time required before the equilibrium temperature is overstepped and run temperature is reached.
A very small temperature gradient (ca. < 3°) in the sample capsule can result in an absence of fluid convection reducing the mineral kinetics. If the diffusion is slow, the composition of the fluid phase now depends on the spatial distribution of the different mineral phases. Pores between dissolving carbonates will contain a fluid phase, which becomes rich in CO2 compared to fluid-filled pores enclosed in silicate aggregates. A system of different microsystems will produce different fluid compositions, which can be studied from fluid inclusions in the product phases. In addition, due to a local accumulation of Ag from decomposed silver oxalate (for producing a specific XCO2), a zoning of composition, pH, and fO2 of the fluid phase is likely (and can be observed, e.g., by different coloured reaction products: Fe2+/Fe3+). Therefore, calculations of activation energies from kinetic experiments with small temperature gradients must consider these various experimental factors.
Finally, low concentrations of additional species in the fluid phase, which do not participate in the overall reaction, can influence the elementary reactions, like dissolution or transport. In particular, chloride contents (CaCl2 or NaCl, which are often described from fluid inclusion investigations of metamorphic rocks) in the H2O-CO2-fluid phase strongly control the mechanism and consequently the kinetic of the reaction. Even low chloride concentrations (we used 0.1 m NaCl and 0.05 m CaCl2 solutions), which have no thermodynamic influence (< 0.1°C) on the equilibrium temperature of 615°C (500 MPa), do effect the kinetic of the reaction significantly. In comparison with non-saline fluids the rate of the diopside-forming reaction is doubled using NaCl solution and is increased even by a factor of 3 to 4 with CaCl2 solution. Additional experiments with rock cylinders using chloride solutions indicate that the mobilisation and the transport of the dissolved species in the rock samples are spatially expanded producing textures, which differ from those in rock cylinder experiments with chloride-free solutions. These simple conditions become even more complicated using stronger chloride solutions. The unmixing of the fluid phase in the experiments at higher temperatures will produce a reactive saline brine and a less reactive CO2-rich fluid phase. Due to different separation possibilities textural and compositional zoning can occur influencing the kinetics and the mechanism of the overall reaction.
Lüttge, A. & Metz, P., Can. Mineral. 29, 803- 821 (1991).
Lüttge, A. & Metz, P., Contr. Mineral. Petrol. 115, 155-164 (1993).
Neumann, U. & Lüttge, A., Eur. J. Mineral. (submitted).